Silver, who recently launched his new website, FiveThirtyEight.com, said Republicans have a 70 percent chance of unseating Pryor, one of the more vulnerable Democratic incumbents.

"We thought [Nate] Silver made a mistake by giving Pryor such a low [chance]," Schumer said. "That's not our data. In fact, he has fallen less since the Obamacare introduction than any of our other candidates. These are good polls."

The Senate Democrats' campaign arm has sought to downplay Silver's projection that Republicans are the "slight favorite" to win control of the chamber, drawing charges of hypocrisy from Silver.

"Our candidates are very good. In our polling data, we're ahead in all four [states]," Schumer said, referring to Arkansas, North Carolina, Louisiana and Alaska -- the four states with arguably the most vulnerable Democratic incumbents this cycle.